Saw branching filter with a branching filter circuit formed on the package

ABSTRACT

A surface acoustic wave duplexer having an antenna terminal, a transmitting terminal and a receiving terminal. The surface acoustic wave duplexer includes a transmitting SAW filter coupled between the antenna terminal and the transmitting terminal, a receiving SAW filter coupled between the antenna terminal and the receiving terminal, a common piezoelectric substrate on which both of the transmitting SAW filter and the receiving SAW filter are formed, a package covering the common piezoelectric substrate. The antenna terminal, the transmitting terminal and the receiving terminal are formed on the package, and a frequency adjusting circuit is coupled between the antenna terminal and the transmitting SAW filter or the receiving SAW filter. The frequency adjusting circuit has a capacitance element.

This nonprovisional application is a divisional of U.S. application Ser.No. 09/305,304, filed May 5, 1999 now U.S. Pat. No. 6,222,426.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a branching filter that uses an surfaceacoustic wave (SAW) resonance type filter used for compact mobilecommunication equipment for portable telephones and the like.

2. Description of Related Art

In recent years, advances have been made in the development of terminalsfor compact, light mobile communication equipment such as portabletelephones. RF (Radio Frequency) filters are incorporated into theseterminals. Surface acoustic wave (SAW) resonance type filters are usedfor this RF filter.

Accompanying the development of these terminals is a demand for theparts to be made more compact and to have higher performance. Therefore,there is also a demand for more compact, higher performance SAWresonance filters (also called SAW elements).

FIG. 9 is a block diagram of a structural example of a conventionalportable telephone branch filter.

The branch filter 10 shown in FIG. 9 comprises an antenna (ANT) terminal11, an LC chip 12, a transmission filter 13, an Rx-branching filtercircuit strip line 14, a receiving filter 15, a transmission (Tx)terminal 16, and a receiving (Rx) terminal 17. The LC chip 12 isprovided between the ANT terminal 11 and ground. The transmission filter13 is connected between ANT terminal 11 and Tx terminal 16, and theseries circuit of the Rx-branching filter circuit strip line 14 and thereceiving filter 15 are connected in this sequence between the ANTterminal 11 and the receiving terminal 17.

FIG. 10 is a circuit structural figure of the specific circuit structureof the branching filter shown in FIG. 9. Reference number 11 is the ANTterminal, 2, 3, and 4 are branching filter circuit strip lines(inductance) (equivalent to 14 in FIG. 9), 13 is the transmissionfilter, 15 is the receiving filter, 16 is the transmission (Tx)terminal, and 17 is the receiving (Rx) terminal.

Conventionally, with this type of portable telephone branching filter,the transmission and receiving filters were each composed usingdielectric resonators.

FIGS. 11 and 12 show a portable telephone branching filter and mountingaspect, respectively. FIG. 11 is a schematic perspective view of thefront surface, and FIG. 12 is a schematic perspective view of the backsurface.

As is clear from the structural examples shown in FIGS. 11 and 12, chips13 and 15 of the transmission and receiving filters are incorporatedinto on-board substrate 9. Branching filter circuit strip lines 2, 3,and 4 are provided as structural elements on this on-board substrate 9.As is also clear from FIG. 12, this on-board substrate 9 comprises aninsulation substrate 9 a such as a resin substrate, low temperaturesinter substrate, or aluminum substrate, a metallized conductive coatingpattern 9 b provided thereon, and an insulation pattern 9 c formed byexposing substrate 9 a. Branching filter circuit strip lines 2, 3, and 4are formed in continuum with conductive coating pattern 9 b.

With this type of portable telephone branching filter, each chip of thetransmission filter 13 and the receiving filter 15 is provided dividedon separate piezoelectric substrates. Then, these two piezoelectricsubstrates are individually incorporated into one on-board substrate 9,offering the merit of excellent insulation characteristics for bothfilters.

However, besides the piezoelectric substrate on which are provided thechips of these transmission and receiving filters 13 and 15, an on-boardsubstrate 9 with a space for incorporating the Rx-branching filtercircuit strip line 14 and the LC chip 12 (thus, branching filter circuitstrip lines 2, 3, and 4 shown in FIG. 11) is needed, so the on-boardsubstrate becomes large, and the connecting wiring for the branchingfilter structure becomes long. Because of this, the structure of thison-board substrate 9 becomes complex, and the area occupied by theconnecting wiring increases. This inhibits making the on-board substrateand thus the branching filter more compact.

On the other hand, a branching filter has been developed that uses a SAWresonator for the transmission filter and receiving filter (JapanesePatent Laid-open No. 6-97761). For the branching filter that uses a SAWresonance type filter disclosed in this publication, the transmissionand receiving filters comprise ladder-type resonator filters with astructure similar to a serial arm SAW resonator and a parallel arm SAWresonator. With this conventional branching filter, it is possible tomake the branching filter more compact to some degree, but the problemof insulation between filters has not been looked into yet. FIG. 13 is aschematic perspective view showing a structural example of the branchingfilter disclosed in this Japanese Patent Laid-open No. 6-97761. Thisbranching filter has a structure with which the structural elements areincorporated into package 20. Specifically, inside a packageconstruction 21A is provided a ground layer 21B, an impedance matchingelement 22, a phase adjustment element 23, a trap circuit 24, atransmission SAW filter element 25, and a receiving SAW filter element26.

In this way, the conventional branching filter disclosed in JapanesePatent Laid-open No. 6-97761 is structured to house in a single package20 the transmission SAW filter element 25, the receiving SAW filterelement 26, an LC chip, and an Rx-branching filter circuit strip line.

However, in this case, particularly because the transmission SAW filterelement 25, the receiving SAW filter element 26, the LC chip (phaseadjustment element 23), and the Rx-branching filter circuit strip line(impedance matching element 22) are housed within the same package 20,there are problems including a degradation of the insulationcharacteristics between the transmission area and receiving area, and adegradation of the branching filter characteristics as an interactionworks between the connecting wires.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a branchingfilter that uses a SAW resonance type filter that is capable of beingmade more compact as well as having a higher performance level.

Another object of the present invention is to provide branching filterthat uses a SAW resonance type filter with a structure with which thetransmission SAW filter and receiving SAW filter can be placed in onechip.

To achieve these objects, the branching filter of the present inventioncomprises the unique structure described below. Specifically, thebranching filter of the present invention comprises a SAW resonator. TheSAW resonator comprises a transmission SAW filter linked between anantenna terminal and transmission terminal, a receiving SAW filter withdifferent bandpass characteristics from the above-mentioned transmissionSAW filter linked between the above-mentioned antenna terminal andreceiving terminal, and a composite circuit of a frequency adjusting LCcircuit and branching filter circuit (also called a strip line for abranching filter circuit) linked between the above-mentioned antennaterminal and the above-mentioned transmission and receiving SAW filters.Also, with the present invention, this branching filter circuit iscomposed from a serial arm SAW resonator. Specifically, all or part ofthis branching filter circuit is structured as a serial arm SAWresonator.

For a preferred embodiment of the present invention, it is desirable tohave a structure, between the antenna terminal and transmission SAWfilter, with a frequency adjusting LC circuit connected to the antennaterminal and a Tx-branching filter circuit strip line connected as abranching filter circuit between the above-mentioned LC circuit andtransmission SAW filter for the composite circuit. It is also acceptableto have a structure with only a frequency adjusting LC circuit connectedbetween the antenna terminal and transmission SAW filter for thecomposite circuit.

For a preferred embodiment of the present invention, it is desirable tohave a structure, between the antenna terminal and receiving SAW filter,with the frequency adjusting LC circuit described above connected to theantenna terminal and an Rx-branching filter circuit strip line connectedas a branching filter circuit between the aforementioned LC circuit andreceiving SAW filter as the composite circuit.

For a preferred embodiment of the present invention, it is desirable toform a single piezoelectric substrate shared by the transmission SAWfilter and receiving SAW filter.

With a preferred embodiment of the present invention, it is desirable toform a single piezoelectric shared by the transmission SAW filter, thereceiving SAW filter, and the branching filter circuit.

With another preferred embodiment of the present invention, it isdesirable to form a single piezoelectric substrate shared by thetransmission SAW filter, the frequency characteristics adjusting LCelement, the Rx-branching filter circuit strip line, and the receivingSAW filter.

With yet another preferred embodiment of the present invention, it isdesirable to form a single piezoelectric substrate shared by thetransmission SAW filter, the receiving SAW filter, and the branchingfilter circuit, and to provide a frequency adjusting LC element outsideof the piezoelectric substrate.

With yet another preferred embodiment of the present invention, it isdesirable to provide on the on-board substrate a piezoelectric substrateon which in some cases the transmission SAW filter and receiving SAWfilter are formed together with the branching filter circuit and/orfrequency adjusting LC element.

Also, for this preferred embodiment, it is desirable to form a singlecombined SAW resonator from a first level (stage) serial arm SAWresonator on the antenna terminal side and a serial arm SAW resonator ofthe branching filter circuit for both or only one of the transmissionSAW filter and the receiving SAW filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will be better understood from the following description takenin connection with the accompanying drawings, in which;

FIG. 1 is a block diagram that gives a summary explanation of astructural example of the branching filter using a SAW resonator typefilter of the present invention;

FIG. 2 is a circuit block diagram for explaining a specific structuralexample of the branching filter using a SAW resonator type filter of thepresent invention;

FIG. 3 is a circuit block diagram for explaining another specificstructural example of the branching filter using a SAW resonance typefilter of the present invention;

FIG. 4 (including FIGS. 4(A) through 4(C)) is a schematic obliquediagram for explaining an aspect of the branching filter using a SAWresonance type filter of the present invention;

FIG. 5 is a block diagram for explaining the function of each structuralelement of the branching filter when the transmission operation isperformed for the branching filter using a SAW resonance type filter ofthe present invention;

FIG. 6 is a block diagram for explaining the function of each structuralelement of the branching filter when the receive operation is performedfor the branching filter using a SAW resonance type filter of thepresent invention;

FIG. 7 is a figure that provides an explanation of the impedance of thebranch filter using a SAW resonance type filter of the presentinvention;

FIG. 8 (including FIGS. 8(A) and 8(B)) is a figure that provides anexplanation of the serial arm SAW resonator and its LC equivalentcircuit used for the branching filter using a SAW resonance type filterof the present invention;

FIG. 9 is a block diagram that gives a summary explanation of astructural example of a conventional branching filter using a SAWresonance type filter;

FIG. 10 is a figure that explains a specific structural example of aconventional branching filter using a SAW resonance type filter;

FIG. 11 is a schematic perspective view seen from the front side forexplaining an aspect of a conventional branching filter using a SAWresonance type filter;

FIG. 12 is a schematic perspective view seen from the back side forexplaining an aspect of a conventional branching filter using a SAWresonance type filter; and

FIG. 13 is a schematic perspective view seen from the front side forexplaining another aspect of a conventional branching filter using a SAWresonance type filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, a detailed explanation will be given topreferred embodiments of the branching filter of the present invention.In the drawings, the structural elements are simply shown in summaryform to make the invention easier to understand. FIG. 1 is a blockdiagram that schematically shows a structural example of the branchingfilter using a SAW resonator type filter of the present invention.

The branching filter 100 in the structural example shown in FIG. 1comprises an antenna terminal 102, a transmission terminal 104, and areceiving terminal 106. Also, this branching filter 100 comprises atransmission SAW filter 140 linked or connected between the antennaterminal 102 and the transmission terminal 104 and a receiving SAWfilter 150 linked or connected between this antenna terminal 102 and thereceiving terminal 106. This transmission SAW filter 140 and receivingSAW filter 150 have different bandpass characteristics from each other.Furthermore, the branching filter 100 comprises a composite circuit 160made from a frequency adjusting LC circuit 108 and a branching filtercircuit 110 between this antenna terminal 102 and each of thetransmission SAW filter 140 and the receiving SAW filter 150. Thesetransmission and receiving SAW filters 140 and 150, the frequencyadjusting LC circuit 108, and the branching filter circuit 110 form abranching filter circuit that uses a SAW resonator type filter.

Also, with the present invention, as will be described later withreference to FIGS. 2 and 3, part of this branching filter circuit 110 isconstructed from a serial arm SAW resonator.

Preferably, the branching filter circuit 110 comprises a transmissionside Tx-branching filter circuit strip line 120 and a receiving sideRx-branching filter circuit strip line 130. However, the Tx-branchingfilter circuit strip line 120 is not absolutely necessary, so it can beused as appropriate for specific designs.

Therefore, it is acceptable to construct the composite circuit 160,between the antenna terminal 102 and the transmission SAW filter 140,with the frequency adjusting LC circuit 108 connected to the antennaterminal 102 and the Tx-branching filter circuit strip line 120connected between the aforementioned LC circuit 108 and the transmissionSAW filter 140. And/or, this composite circuit 160 can be constructedonly with the frequency adjusting LC circuit 108 connected between theantenna terminal 102 and the transmission SAW filter 140.

On the other hand, this composite circuit 160 is preferably constructedbetween the antenna terminal 102 and the receiving SAW filter 150 withthe frequency adjusting LC circuit 108 described above connected to theantenna terminal 102 and the Rx-branching filter circuit strip line 130connected between the aforementioned LC circuit 108 and the receivingSAW filter 150.

A specific example of the branching filter 100 described above will beexplained with reference to FIGS. 2 and 3 in a case when the branchingfilter 100 comprises the Tx-branching filter circuit strip line 120 andRx-branching filter circuit strip line 130. FIG. 2 is a circuit diagramthat shows a specific structural example of the branching filter 100using a SAW resonator of the present invention. FIG. 3 is a circuitdiagram that shows another specific structural example of the branchingfilter 100 of the present invention.

In the structural example shown in FIG. 2, the transmission SAW filter140 is constructed as a ladder-type filter made from a two layerstructure of a serial arm resonator and a parallel arm resonator.Specifically, the serial arm, connected between the Tx-branching filtercircuit strip line 120 and the transmission terminal 104, comprises afirst level (first) serial arm resonator (TS1) 140 a from theTx-branching filter circuit strip line 120 side and a second level(second) serial arm resonator (TS2) 140 b. The parallel arm comprises afirst layer (first) parallel arm resonator (TS3) 140 c connected betweenthe first layer and second layer serial arm resonators 140 a and 140 bconnection points and earth and a second layer (second) parallel armresonator (TS4) 140 d connected between the transmission terminal 104and earth.

In comparison, the receiving SAW filter 150 is constructed as aladder-type filter made from a three layer structure serial armresonator and parallel arm resonator. Specifically, the serial arm,connected between the Rx-branching filter circuit strip line 130 and thereceiving terminal 106, comprises a first layer (first) serial armresonator (RS1) 150 a from the Rx-branching filter circuit strip line130 side, a second layer (second) serial arm resonator (RS2) 150 b, anda third layer (third) serial arm resonator (RS3) 150 c. The parallel armcomprises a first layer parallel arm resonator (RP1) 150 d connectedbetween the connection point of first layer and second layer serial armresonators 150 a and 150 b and earth, a second layer (second) parallelarm resonator (RP2) 150 e connected between the connection point ofsecond and third serial arm resonators 150 b and 150 c and earth, and athird layer (third) parallel arm resonator (RP3) 150 f connected betweenreceiving terminal 106 and earth.

With the structural example shown in FIG. 2, from the perspective ofmaking the branching filter and therefore the SAW resonator filter morecompact, the branching filter circuit strip lines 120 and 130 arerespectively composed from the serial arm resonators (TxS and RxS) 120 aand 130 a.

In FIG. 2, the frequency adjusting LC circuit 108 comprises a capacitorcomponent 108 a and an inductor component 108 b which exist between theantenna terminal 102 and the branching filter circuit 110, and thereforebetween the Tx-branching filter circuit strip line 120 and theRx-branching filter circuit strip line 130. The capacitance of thiscapacitor component 108 a is C_(ANT), and the inductance of the inductorcomponent 108 b is L_(ANT).

With the present invention, as shown by the structural example shown inFIG. 2, it is also acceptable to provide the serial arm resonators 120 aand 130 a for branching filter circuit strip lines described above andthe transmission and receiving SAW filter first level serial armresonators 140 a and 150 a individually.

However, to make the resonator filter more compact, it is alsoacceptable to combine these two transmission side serial arm resonators120 a and 140 a to construct a single composite or combined resonator.Similarly, it is also acceptable to combine these two receiving sideserial arm resonators 130 a and 150 a to construct a single composite orcombined resonator. FIG. 3 shows a structural example with these serialarm resonators 120 a and 140 a combined into a composite resonator 142and serial arm resonators 130 a and 150 a combined into a compositeresonator 152. The other structural elements shown in FIG. 3 areconstructed in the same manner as the structural example shown in FIG.2.

However, with the present invention, as has already been explained, thegoal is to achieve a more compact branching filter with a higherperformance level by combining the transmission and receiving filtersinto one chip. To do this, in addition to the mechanism of the circuitstructure described above, if possible, it is also necessary to have amechanism for surface mounting of the structural elements that form thebranching filter.

FIG. 4 (including FIGS. 4(A) through 4(C)) is a schematic perspectiveview that explains a structural example seen from the perspective of anaspect of the branching filter of the present invention.

FIG. 4(A) shows an example of the transmission SAW filter 140 and thereceiving SAW filter 150 formed together on one piezoelectric substrate170. Then, this piezoelectric substrate 170 is incorporated into thepackage on-board substrate 180. A resin substrate, low temperaturesinter substrate, or aluminum substrate can be used as this on-boardsubstrate 180. It is also possible to use a multi-layer substrate forthis on-board substrate. In this case, it is possible to provide thefrequency adjusting LC circuit and branching filter circuit strip lineoutside the piezoelectric substrate 170, one example being on-boardsubstrate 180. In FIG. 4(A), Tx-in and Tx-out are transmission inputterminals and output terminals, and Rx-in and Rx-out are receiving inputterminals and output terminals. Transmission output terminal andreceiving input terminal Tx-out and Rx-in are connected to the antennaterminal 102 (FIG. 1) in on-board substrate 180. On the other hand,transmission input terminal and receiving output terminal Tx-in andRx-out correspond respectively to the transmission terminal 104 and thereceiving terminal 106 shown in FIG. 1.

FIG. 4(B) shows a structural example of the transmission SAW filter 140,the receiving SAW filter 150, and the branching filter circuit 110 beingformed on a single common piezoelectric substrate 170. When theTx-branching filter circuit strip line 120 and the Rx-branching filtercircuit strip line 130 are contained in the branching filter circuitstrip line 110, both can be provided on this piezoelectric substrate170. Or, when only the Tx-branching filter circuit strip line 120 iscontained in the branching filter circuit strip line 110, it isacceptable to provide only this Tx-branching filter circuit strip line120 on the piezoelectric substrate 170. Also, in FIG. 4(B), the requiredwiring and input terminals and output terminals are not illustrated, andthe Tx-branching filter circuit strip line 120 is shown by a dotted linewhile the Rx-branching filter circuit strip line 130 is shown by a solidline. In this structural example, the frequency adjusting LC element 108can be provided outside the piezoelectric substrate 170.

FIG. 4(C) shows a structural example in which the transmission SAWfilter 140, the frequency characteristics adjusting LC element 108, theRx-branching filter circuit strip line 130, and the receiving SAW filter150 are all formed on a single common piezoelectric substrate 170. Whenthe Tx-branching filter circuit strip line 120 is contained in thebranching filter circuit strip line 110, this Tx-branching filtercircuit strip line 120 can be provided on the piezoelectric substrate170. Also, in this FIG. 4(C), required wiring and input terminals andoutput terminals are not illustrated, and this Tx-branching filtercircuit strip line 120 is shown as a dotted line.

In this way, the transmission SAW filter 140 and the receiving SAWfilter 150, or in some cases, the branching filter circuit 110 and/orthe frequency adjusting LC element 108 are formed together on thepiezoelectric substrate 170 (the piezoelectric substrate shown on any ofFIGS. 4(A) through 4(C)), and this can be provided on the on-boardsubstrate 180.

The Tx-branching filter circuit strip line 120 and the Rx-branchingfilter circuit strip line 130 formed on this piezoelectric substrate 170are each composed from a serial arm SAW resonator. Also, the structuralelements provided outside the piezoelectric substrate 170 (in thestructural example in FIG. 4(A), the branching filter circuit and thefrequency adjusting LC element, or in the structural example in FIG.4(B), the frequency adjusting LC element) are provided in a package thathouses the on-board substrate 180. Or, though not illustrated, thepackage can be formed with a multi-layer structure, and the structuralelements provided outside the piezoelectric substrate 170 can beprovided on the intermediate layer or the upper layer (including thepackage lid). By using a structure like those of the structural examplesshown in FIGS. 4(A) through (C), it is possible to make branchingfilters more compact and give them a higher performance level.

Next, explanation will be given to an example of operation of abranching filter using a SAW resonator of the present invention. FIG. 5is a structural diagram that shows this branching filter by individualfunction during a transmission operation. FIG. 6 is a structural diagramthat shows this branching filter by individual function during a receiveoperation. FIG. 7 provides an explanation of the impedance of thisbranching filter.

The branching filter 100 handles transmission and receiving by oneantenna 200. To do this, the transmission system and receiving systemare directly connected to the antenna. Therefore, the performance ofthis branching filter 100 is largely related to the performance ofportable telephones.

As shown in FIG. 5, when the branching filter 100 is used fortransmission, transmission signals from a power amplifier 210 are sentto the transmission filter 140 via the transmission terminal 104. Thefrequency band of these transmission signals are restricted by thetransmission filter 140, are sent to the antenna 200 via the antennaterminal 102, and transmission signals are sent from here. In this case,the receiving system 220 that contains the Rx-branching filter circuitstrip line 130 and the receiving filter 150 is viewed as a load circuittogether with the antenna 200.

Also, as shown in FIG. 6, when the branching filter 100 is used forreceiving, signals received by the antenna 200 are sent to the receivingfilter 150 via the antenna terminal 102. With the receiving filter 150,the frequency band of the received signals is restricted, and these aresent to the receiving circuit 220 via the receiving terminal 106. Inthis case, the transmission system 230 that includes the Tx-branchingfilter circuit strip line 120 and the transmission filter 140 is viewedas a load circuit together with the antenna 200.

In view of these points, the necessary conditions for a branching filterto function as a high performance branching filter are as follows.

The input impedance on the side of the Rx that includes a branchingfilter circuit when using the branching filter for transmission (FIG. 5)is Zr. This Zr is shown as 400 in FIG. 7. This Zr must satisfy theconditions of the following approximate expressions (1-1) and (1-2).

 Zr*Z _(ANT)/(Zr+Z _(ANT))≈50  (1-1)Zr≈∞  (1-2)

The input impedance on the side of the Tx that includes a branchingfilter circuit when using the branching filter for receiving (FIG. 6) isZt. This Zt is shown as 300 in FIG. 7. This Zt must satisfy theconditions of the following approximate expressions (2-1) and (2-2).Zt*Zr/(Zt+Zr)≈50  (2-1)Zt≈∞  (2-2)

For portable telephones, the transmission band is 890 to 915 MHz and thereceiving band is 935 to 960 MHz. With the transmission filter 140 inthe transmission system 230 shown in FIG. 6, it is possible to set theend frequency to the receiving band of 930 to 960 MHz using the serialarm SAW resonator of this filter, so the transmission filter 140 in thiscase can satisfy the input impedance approximate expression (2-1).However, for the transmission system 230, it is not possible to set theend frequency of the serial arm SAW resonator of this filter to thetransmission band of 890-915 MHz. Because of this, it is not possible tosatisfy the input impedance approximate expressions (1-1) and (1-2).

FIG. 8(A) is a circuit diagram showing the serial arm SAW resonator usedfor the branching filter of the present invention, and FIG. 8(B) is anLC equivalent circuit diagram of this resonator.

Thus, to compare the impedance characteristics during transmission for abranching filter of a conventional structure (FIG. 9) and those for thebranching filter of the present invention (FIG. 5), a simulation wasperformed. The branching filters used as the subject of this simulationwere GSM method branching filters that use a portable telephone type SAWresonator. This GSM method branching filter does not comprise thestructural elements shown by 120 a and inductor 108 b in FIG. 2, butrather has a structure comprising the Rx-branching filter circuit stripline (for the conventional branching filter) or in place of this theserial arm SAW resonator 130 a (for the branching filter of the presentinvention). Also, of the frequency band 890 to 960 MHz, the simulationwas performed at 890, 915, 935, and 960 MHz.

The GSM method branching filter transmission filter 13 of conventionalmethods and transmission filter 140 of the present invention used assubjects both have the same structure as the transmission filter shownby 140 in FIG. 2. Similarly, the receiving filters 15 and 150 both havethe same structure as the receiving filter shown by 150 in FIG. 2. Table1 shows the intersection length (shown as D (μm) in table 1) andelectrode logarithm (shown by M in table 1) of the SAW resonator thatcomposes the transmission and receiving filters of these branchingfilters. (Please refer to the attached table 1.) In table 1, the SAWresonators 140 a, 140 b, 140 c, and 140 d that compose the transmissionfilter 140 shown in FIG. 2 are shown as TS1, TS2, TS3, and TS4. Also,the serial arm SAW resonators 150 a, 150 b, and 150 c that compose thereceiving filter 150 in FIG. 2 are shown as RS1, RS2, and RS3. Also,parallel arm SAW resonators 150 d, 150 e, and 150 f are shown as RP1,RP2, and RP3. Furthermore, with the branching filter of the presentinvention used as a subject for simulation, the Rx-branching filtercircuit strip line 130 (FIG. 1) has been substituted by the serial armSAW resonator 130 a, so this serial arm SAW resonator 130 a is shown asRxS. Note that the Tx-branching filter circuit strip line 120 and theserial arm SAW resonator 120 a that should be substituted for this(shown as TxS in FIG. 2) have been omitted.

Furthermore, for the conventional branching filter (shown as 10 in FIG.9) that is the subject of simulation, the transmission filter 13 isincorporated into a single piezoelectric substrate, receiving filter 15is incorporated into another single piezoelectric substrate, and theRx-branching filter circuit strip line 14 and the LC chip 12 areprovided on a multi-layer substrate (on-board substrate) for which thesetransmission and receiving filters 13 and 15 are incorporated on theabove-mentioned piezoelectric substrates.

Table 2 shows branching filter circuit impedance values for the desiredparameters and obtained simulation results for the specific type ofbranching filter circuit. In Table 2, code items I and II indicate abranching filter of a conventional structure. Code items III, IV, and Vindicate a branching filter of the present invention. (Please refer tothe attached Table 2.)

In this Table 2, with the conventional branching filter 1, the structureis such that the branching filter circuit is the strip line, only theRx-branching filter circuit strip line (strip line length (LR)=40 mm)(shown as 14 in FIG. 9) is provided, without providing the Tx-branchingfilter circuit strip line (strip line length (LT)=0 mm), and there isalso no frequency adjusting LC chip (shown as 12 in FIG. 9) provided.Therefore, the input terminal of the transmission filter 13 and theinput terminal of the Rx-branching filter circuit strip line 14 aredirectly connected to the antenna terminal 11.

Also, with the conventional branching filter II, the structure is suchthat the branching filter circuit is the strip line, neither theTx-branching filter circuit strip line nor the Rx-branching filtercircuit strip line is provided (strip line length (LT, LR)=0 mm), andthere is also no frequency adjusting LC chip (shown as 12 in FIG. 9)provided. Also, the input terminals of the transmission filter 13 andthe receiving filter 15 are directly connected to the antenna terminal11.

The three types of branching filter of the present invention III, IV,and V that are subjects of simulation are not provided with aTx-branching filter circuit strip line 120 or a serial arm SAW resonator(TxS) 120 a (FIG. 2) in the circuit structure shown in FIG. 1.Therefore, the input terminal of the transmission filter 140 is directlyconnected to the frequency adjusting LC element 108. Furthermore, thesebranching filters III, IV, and V have a structure with which instead ofproviding the Rx-branching filter circuit 130, the serial arm SAWresonator (RxS) 130 a is provided. Then, as has already been explainedin reference to FIGS. 2 and 3, these branching filters III, IV, and Vare structured with the serial arm SAW resonator 130 a, and the firstlevel serial arm SAW resonator 150 a of the receiving filter 150combined as a composite resonator 152. Based on such conditions, thebranching filter III comprises capacitor component C_(ANT)4(capacitance=10 pF) and inductor L_(ANT) (inductance=7 nH) as theexternally mounted frequency adjusting LC element 108. Also, thebranching filter IV comprises not capacitor component C_(ANT) but onlyinductor L_(ANT) (inductance=7 nH) as the externally mounted frequencyadjusting LC element 108. Similarly, the branching filter V comprisesnot capacitor component C_(ANT) but only inductor L_(ANT) (inductance=10nH) as the externally mounted frequency adjusting LC element 108. Inthis way, the branching filter of the present invention is structuredwith the goal of improving frequency characteristics using the frequencycharacteristics adjusting LC element 108. Impedance values for 890, 915,935, and 960 MHz are shown for the transmission and receiving filters ofthe conventional technology and the present invention.

Of the conventional branching filters and that of the present invention,Table 3 shows real number and imaginary number values for inputimpedance Zt 300 of the transmission filter 140 and input impedance Zr400 of the receiving filter 150 as shown in FIG. 7 for specifiedbranching filters, specifically branching filters II (conventional) andIV (the present invention) of Table 2. Then, input impedance values for890, 900, 915, 935, and 960 MHz are shown for the transmission andreceiving filters. (Please refer to the attached Table 3.)

By comparing input impedance Zt and Zr of branching filters II and IV inTable 3, it became clear that the receiving filter transmission bandimpedance is large for the branching filter of the present invention.Looking specifically, for the receiving filter of Table 1, whenfrequency f is 890 MHz, the input impedance Zr for branching filter IIhas a real number of 0.0127 and an imaginary number of −1.098. Incomparison, the input impedance Zr for branching filter IV has a realnumber value of 3.54 and an imaginary number value of 23.20. In thisway, the input impedance of the branching filter of the presentinvention is significantly larger than that of the conventionalbranching filter. Thus, with the branching filter of the presentinvention, we can see that there is a significant improvement infrequency characteristics. This improvement is also clear from theimpedance characteristics results shown in Table 2.

Also, from the impedance characteristics results of table 2, we can seethat the transmission frequency band is 890 to 915 MHz and the receivingfrequency band is 935 to 960 MHz.

As has already been explained, the branching filter of the presentinvention which was provided for the above-mentioned impedancecharacteristics simulation has a structure made more compact byincluding a receiving filter first level serial arm SAW resonator in theRx-branching filter circuit strip line. Here, we will explain the inputimpedance at f=900 MHz which is the central frequency of thetransmission band which is of the most interest in terms of portabletelephone quality.

When we see the transmission filter side from the point C 500 shown inFIG. 7, the combined impedance Z_(IN) (Tr) is given by the followingequation (3).Z _(IN)(Tr)=Zt*Zr/(Zt+Zr)  (3)

In this case, the input impedance of the transmission filter 140 and thereceiving filter 150 at f=900 MHz is as follows based on Table 3.Zt(900)=0.863−j0.626  (4)Zr(900)=0.0175−j0.934  (5)

Therefore, impedance Z_(IN) (Tr) (900) on the transmission filter sidefrom the point C 500 shown in FIG. 7 is given by the following equation(6).Z _(IN)(Tr)(900)=0.2409−j0.501  (6)If this Z_(IN) (Tr) (900) undergoes impedance correction only by theinductance L_(ANT) 108 b of the frequency adjusting LC element 108 inthe structural example shown in FIG. 2 for the present invention, thenthe value of the inductance L_(ANT) is as follows. L _(ANT)=4.4(nH)  (7)In this case, when the characteristics impedance is not the desiredvalue, an impedance matching circuit must be introduced.

In reality, with this type of portable telephone, the optimumcharacteristics are demanded not only for frequency f=900 MHz but forthe transmission band (890 to 915 MHz). These optimum characteristicsare normally determined using simulation. The branching filters IV and Vshown in Table 2 show the results of adjusting impedance fortransmission band 890 to 915 MHz using only the inductor L_(ANT) 108 b.Also, the branching filter III shown in Table 2 shows the results ofadjusting impedance for the same kind of transmission band using theinductor L_(ANT) 108 b and the capacitor C_(ANT) 108 a. One combinationof these inductor L_(ANT) and capacitor C_(ANT) values is L_(ANT)=7.0 nHand C_(ANT)=10.0 pF.

In this way, as is clear from the results shown in Table 2, with thestructure of the branching filter of the present invention, byincorporating the transmission filter 140 and the receiving filter 150on a single common piezoelectric substrate and by providing anexternally mounted frequency adjusting LC circuit 108, it became clearthat it is possible to improve frequency characteristics, particularlypassband characteristics.

From the results of Table 3, it can be found that the impedance for thetransmission band of the receiving filter 150 is large using thebranching filter circuit strip line serial arm SAW resonator 130 a. Thisimprovement in frequency characteristics depends on the impedance seenon the filter side from the point C 500 shown in FIG. 7. Specifically,it is assumed that this is caused by introducing the branching filtercircuit strip line serial arm SAW resonator 130 a (RxS) on the inputterminal of the receiving filter 150. Inconsistencies in impedancevalues due to the introduction of this branching filter circuit stripline serial arm SAW resonator 130 a (RxS) are adjusted with anexternally mounted frequency adjusting LC element 108. This frequencyadjusting LC element is made into chip form, and by providing this LCchip on the on and receiving filter package on-board substrate or byproviding it on a piezoelectric substrate on which is created atransmission and receiving filter, it is possible to make a branchingfilter comprising a SAW resonator generally more compact and higher inperformance.

It will be clear to those in the industry that it is possible to makemany changes and variations of the present invention without strayingfrom the main point of the present invention and without beingrestricted by the preferred embodiments described above.

TABLE 1 TRANSMISSION TS1 TS2 TS3 TS4 FILTER D (μm) M D (μm) M D (μm) M D(μm) M 85 90 42.5 90 84 86 60 60 RECEIVING SERIAL RxS RS1 RS2 RS3 FILTERARM D (μm) M D (μm) M D (μm) M D (μm) M 124 90 124 90 62 90 62 90PARALLEL RP1 RP2 RP3 ARM D (μm) M D (μm) M D (μm) M 102 120 102 120 7680

TABLE 2 BRANCHING FILTER CIRCUIT FREQUENCY TRANSMISSION RECEIVINGADJUSTING LC ELEMENT FILTER FILTER I LT = 0 (mm) LR = 40 (mm) 1.22 1.1735.7 36.8 31.6 58.5 3.11 3.04 II LT = 0 (mm) LR = 0 (mm) 3.56 3.21 37.928.6 33.0 55.6 3.11 2.32 III LT_(ANT) = 7 (nH) C_(ANT) = 7 (pF) 1.281.28 36.6 35.0 34.1 59.0 3.28 3.20 IV LT_(ANT) = 7 (nH) 1.30 1.32 34.733.2 35.1 58.5 3.74 4.0  V LT_(ANT) = 10 (nH) 1.37 1.08 36.1 29.2 35.554.7 3.10 3.70

TABLE 3 TRANSMISSION FILTER RECEIVING FILTER FREQUENCY (MHz) 890 900 915935 960 890 900 915 935 960 II REAL NUMBER 1.283 0.8627 1.345 2.3130.0831 0.0127 0.0175 0.0320 0.606 0.7414 IMAGINARY NUMBER −0.816 −0.62560.5287 0.8715 −4.017 −1.098 −0.934 −0.654 −0.017 1.263 IV REAL NUMBER1.283 0.8627 1.345 2.313 0.0831 3.540 4.7507 0.435 0.875 0.2421IMAGINARY NUMBER −0.816 −0.6256 0.5287 0.8715 −4.017 23.20 0.0479 1.150

1. A surface acoustic wave duplexer having an antenna terminal, atransmitting terminal and a receiving terminal, comprising: atransmitting SAW filter coupled between the antenna terminal and thetransmitting terminal; a receiving SAW filter coupled between theantenna terminal and the receiving terminal: a common piezoelectricsubstrate on which both of the transmitting SAW filter and the receivingSAW filter are formed; and a package covering the common piezoelectricsubstrate, wherein the antenna terminal, the transmitting terminal andthe receiving terminal are formed on the package; a frequency adjustingcircuit being coupled between the antenna terminal and the transmittingSAW filter or the receiving SAW filter, wherein the frequency adjustingcircuit has a capacitance element; and a branching filter circuitcoupled between the frequency adjusting circuit and the transmitting SAWfilter or the receiving SAW filter, wherein the branching filter circuitis formed on the package.
 2. A surface acoustic wave duplexer accordingto claim 1, wherein the package has a first layer substrate and a secondlayer substrate, the first layer substrate is disposed on the secondlayer substrate, and the branching filter circuit is formed on the firstlayer substrate or the second layer substrate of the package.
 3. Asurface acoustic wave duplexer according to claim 1, wherein thefrequency adjusting circuit has an inductance element.
 4. A surfaceacoustic wave duplexer according to claim 1, wherein the frequencyadjusting circuit is formed on the package.
 5. A surface acoustic waveduplexer according to claim 1, wherein the capacitance element iscoupled in series between the antenna terminal and the transmitting SAWfilter or the receiving SAW filter.
 6. A surface acoustic wave duplexeraccording to claim 1, wherein the branching filter circuit comprises aserial arm resonator.
 7. A surface acoustic wave duplexer according toclaim 1, wherein the package has a multi-layer structure.
 8. A surfaceacoustic wave duplexer having an antenna terminal, a transmittingterminal and a receiving terminal, comprising: a SAW filter chipincluding a transmitting SAW filter connected with the transmittingterminal and a receiving SAW filter connected with the receivingterminal, wherein both the transmitting SAW filter and the receiving SAWfilter are formed on one common piezoelectric substrate; a packagecovering the one common piezoelectric substrate, wherein the antennaterminal, the transmitting terminal and the receiving terminal areformed on the package; and a frequency adjusting circuit being coupledbetween the antenna terminal and the transmitting SAW filter or thereceiving SAW filter, wherein the frequency adjusting circuit has acapacitance element; and a branching filter circuit being coupledbetween the frequency adjusting circuit and the transmitting SAW filteror the receiving SAW filter, wherein the branching filter circuit isformed on the package.
 9. A surface acoustic wave duplexer according toclaim 8, wherein the package has a first layer substrate and a secondlayer substrate, the first layer substrate being disposed on the secondlayer substrate, and the branching filter circuit is formed on the firstlayer substrate or the second layer substrate.
 10. A surface acousticwave duplexer according to claim 8, wherein the frequency adjustingcircuit has an inductance element.
 11. A surface acoustic wave duplexeraccording to claim 8, wherein the capacitance element is coupled inseries between the antenna terminal and the transmitting SAW filter orthe receiving SAW filter.
 12. A surface acoustic wave duplexer accordingto claim 8, wherein the branching filter circuit comprises a serial armresonator.
 13. A surface acoustic wave duplexer according to claim 8,wherein the package has a multi-layer structure.